In heating a material radiant at high temperatures, such as a steel product for example, in a heating furnace, it is a usual practice to heat many materials different in quality, shape and size in the same furnace and at the same time. In such a case, it is necessary to take some measures to definitely discriminate from outside the furnace a specific material as being radiant from among many materials so heated.
The method conventionally employed for this discrimination is to draw a discrimination mark in advance on the surface of the material to be charged into the furnace with a heat resisting paint and then to discriminate the material radiant-heated in the furnace with the naked eyes from outside the furnace.
In a heating furnace using heavy oil or coke as fuel, however, it has been difficult to definitely discriminate the material radiant-heated in the furnace as such from outside the furnace by the above-described conventional method. The reasons are as follows. In a heating furnace using heavy oil or coke as fuel, the radiant-heated material in the furnace and the discrimination mark drawn on its surface are shielded by gases generated therein such as CO.sub.2 and C.sub.2 H.sub.2. Spectra radiant therefrom are not only in a state different from spectra outside the furnace but are also unstable in distribution. Accordingly, however excellent in heat resistance and adherence, the discrimination mark observed from outside the furnace is not clear at all in contrast and does not permit easy discrimination. As a result, it becomes difficult to discriminate from outside the furnace the radiant-heated material in the furnace as being radiant (e.g. heated to redness, yellowness, whiteness, etc.).
An object of this invention is therefore to provide a method of definitely discriminating from outside the furnace a material radiant-heated in a heating furnace as being so heated, as well as of eliminating the disadvantages found in the conventional method as mentioned above.
More specifically, the principal object of this invention is to provide a method of definitely discriminating from outside the furnace a discrimination mark previously drawn on the surface of material radiant-heated in a heating furnace by making use of near ultraviolet rays of a high pressure mercury lamp.